1. Absorption of information restricted; memory impaired; inhibitions lowered2. Tunnel vision; difficulty in distinguishing light intensity 3. Central nervous system impaired; intestinal irritation can lead to ulcers; high levels can lead to coma or death4. Change in fat metabolism, eventually leading to scarring of the liver5. Sexual performance inhibited, possibly leading to impotence6. Co-ordination and motor skills impaired; increased swaying

Source: Johns Hopkins University

It's a familiar feeling. A few too many the night before and the morning brings headaches, nausea - and the pledge that you'll never drink again.

But then you do.

Given that experience tells us drinking will ultimately lead to such symptoms, why do people still do it and what makes alcohol addictive?

The answer is a complex jigsaw of brain chemicals, personality and genetics.

'Tranquiliser'

Many of the clues lie deep within the brain.

Alcohol triggers the release of dopamine - a chemical which produces feelings of satisfaction.

It also increases the production of the brain's natural painkiller - endorphin - which scientists think could be the means by which the brain becomes trained to crave.

In addition, drinking affects the glutamate and GABA, chemicals which control how essential messages are sent between nerve cells in the brain.

Scientists believe this effect is part of the development of tolerance to, and dependence on, alcohol.

But Dr Jonathon Chick, a consultant psychiatrist at Edinburgh University said: "Chemicals are only part of the picture, They do not explain it fully.

"For example, there are people who have a nervous, anxious disposition.

"If they start to use alcohol as a tranquiliser, that can become addictive."

Genetic link

The way to stop people drinking and prevent addiction is simply to put up the price of alcohol

Dr Jonathan Chick, Edinburgh University

Dr David Ball, a researcher for the charity Action on Addiction. added: "There are psychological traits, such as sensation seeking, which are linked to illicit drug use - though there is not a specific addictive personality."

Another part of the explanation lies in a person's genes. Someone whose parents had a problem with alcohol have a 10-fold chance of having a problem themselves compared to someone whose parents did not have a problem.

Researchers are currently investigating which genes are responsible.

Doctors say the increased availability of alcohol means genetic predispositions to addiction - and the biological effects of drinking too much are triggered more often.

Dr Ball said: "People can have a genetic predisposition to alcohol addiction, but it can't be triggered if people aren't exposed to alcohol in the first place."

Dr Crick added: "More people are drinking more often, and so more of the at-risk people end up in difficulties compared to 30 years ago when fewer people drank."

Both doctors welcomed the recent recommendation from the Academy of Medical Sciences which said the cost of alcohol should increase by 10% to encourage people to drink less.

Dr Crick concluded: "The only way to stop people drinking and prevent addiction is to put up the price of alcohol."